Power converters for consumer applications operate at partial load under standby conditions for a relatively large part of their lifetime. Whilst functioning in this operation mode, it is desirable to draw power from the mains supply at as low a level as is conveniently possible. Therefore it is desirable to use a power converter design for the power supply which operates with a high efficiency not only under full load conditions, but also for partial load, especially at low standby mode.
Power supplies operating under nominal “no load” conditions of operation need at least to convert a small power level to supply their own circuitry, such as IC, resistive deciders and opto-couplers. “No load” input power smaller than for example 300 mW and input power smaller then 1 W at 500 mWatts output power is becoming increasingly common as a standard requirement.
For powers larger than approximately 100 Watts at full load, resonant LLC topology is of interest and commonly adopted due to its high efficiency and small volumes/high power density. However, one of the main disadvantages for resonant LLC technology is its relatively low efficiency under low load operation (when operated in the most common operation mode, that is, using a fifty percent duty cycle). Losses in this mode of operation may be a multiple of the required stand-by power.
A second mode of operating a resonant power supply under low load conditions is to use “burst mode” operation. In this case the resonant power supply is completely switched off periodically. Whilst switched on, hard switching cannot be avoided. Furthermore large output filters are required to make effective use of burst mode operation.
An alternative solution has been proposed in Patent Application Publication WO 2005/112238A2, to Koninklijke Philips Electronics N.V. This publication discloses a method wherein the timing of the two control switches is such that high side switch (HSS) conducts during a short interval during which both the primary current increases to a certain level and magnetising energy is built up in the transformer. It is during this interval that most of the output current is delivered. At the end of this interval the HSS is turned off, and the low side switch (LSS) is turned on shortly after this moment (the duration of the gap being such as to facilitate soft switch-on of the LSS, as is well known by those skilled in the art). The output current rapidly decreases to 0. The magnetising current starts to resonate in a resonant circuit which is defined by the resonant capacitor, leakage inductance and the magnetising inductance in series. At a moment corresponding to the Nth negative maximum of the magnetising current, the LLS is turned off. N can typically have a value from 0 to several hundred. At that moment the half bridge load is charged by the magnetising current and provides a soft switch-on for the HSS, ready for the next HSS conduction interval.
Advantageously, this method makes it possible to significantly decrease the core losses due to the magnetising current since these losses are, for very low duty cycles, more or less proportional to the switch on time. Since the amplitude of the magnetising current is much smaller than that for the standard fifty percent duty cycle mode of operation, these losses are significantly reduced. The switching losses are further reduced as the magnetising current is beneficially used to charge the half-bridge node.
However, at very low power levels, the benefits achieved by this method are reduced as follows: firstly, at increasingly low power levels, increasingly more resonant cycles are skipped before the LSS is turned off to restart the switching cycle. This is because the energy per switching cycle is constant so, to reduce the power, the time per switching cycle must increase. As the resonating current resonates in the primary side resonant circuit however there will be some leakage resulting in a damping of the resonance. However, to maintain the ability for soft switching, a certain minimum level of magnetising current must still reside in the resonant circuit at the end of the switching period (i.e. the moment when the current is used to facilitate soft switching). So, to allow for the increased damping when the switching period is longer, the value of the magnetising current just after the HSS turn-off moment should be increased. This will lead to larger core losses, since these losses are proportional to the amplitude of the magnetising current raised to the power 2.3. It is clear, therefore, that as the power is further reduced, there will come a time when the losses attributable to the damping outweigh the switching losses which are being avoided by soft switching. In practical solutions a compromise will be found between the switching losses and the magnetising current losses. So in practice, the efficiency for an LLC converter can drop to around fifty to sixty percent or less at power levels below one percent of full load.
A second disadvantage of operating the power converter in this mode is that of its sensitivity to input voltage variations, since during the HSS conduction period the ratio between the current directly transferred to load and current built up in the magnetising inductors decreases as the voltage decreases. This could lead to a situation where no power at all is converted to the load should the input voltage fall to below approximately seventy five percent of its nominal value. Additional circuitry is required to overcome this, with the consequence of further efficiency decreases and increased costs associated with the additional circuitry.
Therefore, there is a continued need for improved methods of operation of LLC resonant power converters.